1. Field of the Invention
This invention relates to antennas and antenna feed systems. More particularly, this invention pertains to phase monopulse feeds for tracking antennas that are used for tracking a satellite, aircraft, or other target or source of electromagnetic radiation and that may also be used for transferring data to or from the satellite, target or source.
2. Description of the Prior Art
By appropriate design of the feed for a reflector antenna, the feed, in combination with the reflector, can be constructed so as to produce an antenna radiation pattern in the form of a "sum" pattern. The "sum" pattern can receive electromagnetic energy from the reflector and deliver this energy to the "sum" port of the feed. If properly constructed, the same feed can also provide an antenna radiation pattern in the form of a "difference" pattern or patterns which can receive electromagnetic energy from the reflector and deliver this energy to the "difference" port(s) of the feed.
Many different feed systems have been used for this purpose. For instance, an array of discrete elements such as dipoles, or crossed dipoles, slots or horns have been used for this purpose. See e.g. U.S. Pat. No. 5,025,493. Typically an array of five elements in the form of a cross is used. The center element provides the sum pattern and the four peripheral elements provide the difference patterns. One pair of opposing peripheral elements provides a difference pattern that may be used for tracking in one plane. The second pair of opposing peripheral elements provides a second difference pattern oriented at right angles to the first and which may be used for tracking in a second plane, orthogonal to the first. Many other variations of these feed systems have been used for tracking. For example, the right dipole may be "turned off" and the top, bottom and left dipoles excited appropriately to cause the peak of the radiation pattern to be squinted to the left. By appropriate choice of the elements and their excitation, the radiation pattern generated by the feed may be made right-hand or left-hand circularly polarized. If the output from the first difference port is shifted in phase by 90 degrees and combined with the output from the second difference port, the combination may be received from a single "circularly polarized" difference port.
The angular orientation of the radiation pattern generated by the feed system may be altered by various methods, one of the well known methods is the "rho-theta" scanning technique which is described in Y. Choung, K. Goudey, L. Bryans, "Theory and Design of a Ku-Band TE21 Mode Coupler", IEEE Trans. MTT Vol. 30, No. 11, Pg. 1862-1866, November 1982. By appropriate combination of the signals from the circularly polarized sum and difference ports, the peak of the radiation pattern can be made to squint away from the Z axis if a relative phase shift is added to the signal output from the sum or the difference port. If the relative phase shift is varied with time in a predetermined manner, the resultant variations in the output of the combined sum and difference ports may be used to track the target or source.
Another phase mechanism also affects the location of the beam. This phase mechanism is the relative phase of the sum and difference patterns on the Z axis. Ideally the phase difference between these two patterns is a constant over the whole operating band. If the phase difference is constant and if the phase shift introduced by the phase shifter is also constant over the same frequency range, then the peak of the radiation pattern will remain at the same angular coordinate over the entire frequency range. However, if the phase between the radiation patterns varies with frequency, then the phase shift introduced by the phase shifter must be adjusted at each operating frequency in order to keep the radiation pattern pointed in the same direction. As a consequence, for the rho-theta technique to be used in a broadband tracking antenna, it is preferable that the phase relationship between the sum channel radiation pattern and the difference channel radiation pattern on axis be essentially constant over the entire band of frequencies overwhich the feed system is to operate. This constancy of phase simplifies the logic necessary to determine the relationship between the setting of the phase shifter and the position of the peak of the pattern.
Unfortunately, when operated over a wide bandwidth, the performance of a feed consisting of a discrete array of elements degrades substantially because the mutual coupling between the discrete elements varies with the changes in frequency. As a consequence, one or more performance parameters, such as aperture efficiency, sidelobe level, cross talk, boresight shift, error-slope linearity or axial ratio, are significantly degraded.
The combination of a circular waveguide with a coaxially located coaxial waveguide has also been used to provide a sum and difference pattern. Such a design was described in "Dual Band EHF Autotrack Feed", 1990 International Telemetering Coference Proceedings, Volume XXVI, Pg. 241-246. Unfortunately, the phase velocity of the wave within the circular waveguide varies with changes in frequency in a different manner than the variation in phase velocity in the coaxial waveguide. This difference in dispersion substantially degrades the tracking performance of the antenna or conversely requires extension compensation to counteract this effect.